Field Joint Arrangement for a Pipeline and Protecting Method Thereof

ABSTRACT

A method protects a field joint of a pipeline, where chamfered edges of thermally-insulating parent coatings on conjoined pipe lengths are in mutual opposition about a longitudinally-extending gap. The method includes manufacturing an hourglass-shaped inner layer around the pipe lengths, which layer may be moulded. The inner layer extends longitudinally along the gap between the chamfered edges and at least partially overlies the chamfered edges. A thermally-insulating solid insert is assembled from two or more parts to lie in the gap surrounding the inner layer, and pressure is applied radially inwardly from the insert to the inner layer. An outer layer of molten material is manufactured around the insert to form a watertight barrier and to form one or more melted interfaces with the inner layer. Corresponding field joint arrangements are also disclosed.

This invention relates to coated pipelines and in particular toinsulating inserts or infills for field joints of coated rigidpipelines, as used in the subsea oil and gas industry.

Rigid subsea pipelines are commonly formed of lengths of steelpipe—‘pipe joints’—that are welded together end-to-end. Pipe joints aretypically about 12 m in length but may be manufactured in multiples ofthat length, such as double, triple or quadruple pipe joints. Tomitigate corrosion of the pipeline and to insulate the fluids that thepipeline carries in use, pipe joints are pre-coated, when manufactured,with protective and thermally-insulating parent coatings.

In many cases, pipe joints are welded together offshore aboard aninstallation vessel as a rigid pipeline is laid, typically by S-lay orJ-lay methods. It is also common to fabricate pipe stalks of a length of500 m to 1500 m from pipe joints onshore at a spoolbase or yard and thento weld together the pipe stalks end-to-end to spool the prefabricatedrigid pipeline onto a reel. The spooled pipeline is then transportedoffshore for laying in a reel-lay operation. When spooling, bending ofthe pipeline extends beyond elastic limits into plastic deformation thatmust be recovered by subsequent straightening processes duringunspooling when laying the pipe offshore.

It is important to understand that in the subsea oil and gas industry,the terms ‘rigid’ and ‘flexible’ as applied to pipes have clear meaningsthat differ in important respects from general language. For example,nominally ‘rigid’ pipes have enough flexibility to be bent if a minimumbend radius is observed. Yet, such pipes are not regarded in theindustry as being ‘flexible’.

Flexible pipes used in the subsea oil and gas industry are specified inAPI (American Petroleum Institute) Specification 17J and API RecommendedPractice 17B. The pipe body is composed of a composite structure oflayered materials, in which each layer has its own function.

The structure of a flexible pipe allows a large bending deflectionwithout a similarly large increase in bending stresses. The bendinglimit of the composite structure is determined by the elastic limit ofthe outermost plastics layer of the structure, typically the outersheath, which limit is typically 6% to 7% bending strain. Exceeding thatlimit causes irreversible damage to the structure. Consequently, theminimum bending radius or MBR of flexible pipe used in the subsea oiland gas industry is typically between 3 and 6 metres.

Conversely, rigid pipes used in the subsea oil and gas industry arespecified in API Specification 5L and Recommended Practice 1111. Incontrast to flexible pipes, a rigid pipe usually consists of orcomprises at least one pipe of solid steel or steel alloy. However,additional elements can be added, such as an internal liner layer or oneor more outer coating layers. Such additional elements can comprisepolymer, metal or composite materials. Rigid pipe joints are typicallyterminated by a bevel or a thread, and are assembled end-to-end bywelding or screwing them together.

The allowable in-service deflection of rigid steel pipe is determined bythe elastic limit of steel, which is around 1% bending strain. Exceedingthis limit caused plastic deformation of the steel. It follows that theMBR of rigid pipe used in the subsea oil and gas industry is typicallyaround 100 to 300 metres depending upon the cross-sectional dimensionsof the pipe. However, slight plastic deformation can be recovered orrectified by mechanical means, such as straightening. Thus, duringreel-lay installation of a rigid pipeline made up of welded pipe jointsor pipe stalks, the rigid pipeline can be spooled on a reel with atypical radius of between 8 and 12 metres. This implies a bending strainabove 2% for conventional diameters of rigid pipes, requiring thepipeline to be straightened mechanically upon unspooling.

Thermal insulation is an important requirement for many subseapipelines, especially those used to transport crude oil from subseawellheads. As collected at the outlet of a wellhead, crude oil is aviscous, multiphasic, pressurised fluid at an elevated temperature,typically around 100° C. to 180° C. If the crude oil is allowed to coolto a significantly lower temperature, typically below 30° C., somecomponents of the crude oil may solidify by mechanisms such ascoalescence, precipitation or gelling. The waxes, asphaltenes, hydratesor other solid condensates that appear as a result may clog the pipelineand are difficult to remove. Similar issues may arise in subseapipelines used to transport natural gas.

Polypropylene (PP) is most commonly used as the parent coating of pipejoints from which pipelines are fabricated. For example, a three-layerPP (3LPP) coating comprises a first layer of epoxy primer, a second thinlayer of PP bonded to the primer and a third, thicker layer of extrudedPP applied over the second layer. A five-layer PP (SLPP) coating addstwo further layers, namely a fourth layer of PP modified for thermalinsulation, such as glass syntactic PP (GSPP) or a foam, surrounded by afifth layer of extruded PP for mechanical protection of the insulatingfourth layer. Similar additional layers are possible for further thermalinsulation, as in a seven-layer PP (7LPP) coating.

A short length of pipe is left uncoated at each end of a pipe joint tofacilitate welding to abutting pipe joints. After welding, the resultingfield joint comprises two bare steel pipe ends of the abutting pipejoints and the circumferential planar butt weld that joins those pipejoints together. Consequently, the field joint defines a gap in theparent coating that was applied to the pipe joints when they weremanufactured. The end edges of the parent coatings are typically cutback with a tapering angle, typically from 30° to 45°, to definefrusto-conical chamfers that lie in mutual opposition about the plane ofthe weld.

Once the weld between abutting pipe joints passes testing, the fieldjoint must be coated with a field joint coating to mitigate corrosionand to maintain the necessary degree of insulation. Thus, the fieldjoint coating fills the gap in the parent coating. In this respect, itis important that pipelines are covered by continuous thermal insulationextending across the field joints between the successive pipe joints.Otherwise, cold spots may arise that could promote clogging of thepipeline by solid condensates.

A further design constraint of reel-lay pipelines is that the outerdiameter of the field joint coating must be similar to the outerdiameter of the parent coatings on the adjacent pipe joints. Otherwise,the pipeline will be unable to pass through reeling and straighteningequipment or will experience excessive localised stress when doing so.

Field joint coatings may be applied by a casting technique usingthermoset materials such as polyurethane (PU) that cure and harden bycross-linking, or by an injection-moulding technique using moltenthermoplastic materials such as PP that harden by cooling.

In a cast-moulded PU (CMPU) process, an example of which is disclosed inDE 102007018519, the exposed pipe surface at the abutting welded ends ofthe pipe joints is cleaned and a primer is applied. A mould is thenpositioned to enclose the field joint and a two-component urethanematerial is cast into the annular cavity defined within the mould aroundthe field joint. The urethane then cures, cross-linking and solidifyingto form PU in an irreversible chemical reaction. When the PU has curedsufficiently, the mould is removed to leave the field joint coating inplace around the field joint.

Another approach is to use PP as the field joint coating in an injectionmoulded polypropylene (IMPP) process. An example of an IMPP process isdisclosed in our earlier patent application published as WO 2012/004665.

In an IMPP process, the exposed pipe surface at the abutting welded endsof the pipe joints is cleaned, primed and heated, for example usinginduction heating or gas flames. The exposed chamfers at the adjacentends of the parent coatings are also heated to soften them, for exampleusing infra-red heating. The field joint is then enclosed by a mouldthat defines an annular cavity around the field joint. Molten PP isinjected into the cavity under high pressure. Once the PP has cooledsufficiently, the mould is removed, leaving a tube of PP around thefield joint as the field joint coating. This tube is continuous with thetubular parent coating surrounding the pipe joints, such that the sameor compatible coating materials extend all along the length of the pipestring.

A field joint coating of IMPP is advantageous because it has broadlysimilar mechanical and thermal properties to a parent coating of PP.Also, the parent coating and the field joint coating are sufficientlycompatible that they fuse together at their mutual interface, resistingcracking and hence giving longer service life. The service temperatureof PP is also markedly higher than PU.

In the S-lay method, a pipeline is welded from pipe joints along ahorizontal firing line. The pipeline is launched from the vessel over astinger that supports an overbend of the pipeline, from which thepipeline curves down through the water to a sag bend leading to atouchdown point on the seabed. Field joint coating is carried outupstream of the stinger, at one or more coating stations through whichthe pipeline is advanced in stepwise fashion after welding.

Field joint coating is also employed during J-lay installation, in whichpipe joints are lifted into a near-vertical orientation in a tower forwelding to the end of the pipeline. The pipeline hangs near-verticallyfrom the vessel and extends downwardly to a sag bend leading to atouchdown point on the seabed. Field joint coating is carried outdownstream of the welding station in the tower, just before launching anewly-added pipe joint into the sea.

In principle, S-lay allows faster pipelaying than J-lay but J-lay isnecessary in challenging situations where, for example, the subseaenvironment is congested or water depth and strong currents make S-layimpractical without imparting large strains to the pipeline.

The speed of spooling and pipelaying depends upon minimising thetimescale of all operations on the critical path. Given the stepwise,sequential processing steps of welding and field joint coating in S-layand J-lay methods, it is particularly important that neither welding norfield joint coating take longer than is necessary or that one processtakes substantially longer than the other. Otherwise there will be a‘bottleneck’ in the pipeline installation process. Similarconsiderations apply to fabricating a pipeline that is to be spooled forreel-lay.

Moulding a thick field joint coating is among the slowest operationsthat must be performed during stepwise fabrication of subsea pipelines,especially if it is preceded by infra-red heating of the chamfers at thesame workstation. Indeed, heating the chamfers and moulding a fieldjoint coating may together define the cycle time for offshore pipelinefabrication using S-lay and J-lay techniques, including welding andnon-destructive testing (NDT) operations. Accelerating the slowestoperation is the key to reducing the overall cycle time, hence toincrease the speed of pipeline laying.

Similarly, when fabricating pipelines for reel-lay, field joints formedbetween the successive pipe joints and pipe stalks must be coated beforespooling. Thus, welding and field joint coating operations also lie onthe critical path for fabricating pipe stalks and for spooling. In thisrespect, spooling can only take place after a pipe stalk has been weldedcorrectly onto the end of the already-spooled length of pipeline and theresulting field joint has been coated. It follows that delays in weldingand field joint coating operations may also affect reel-lay operations,specifically the time that is required to load a pipeline onto areel-lay installation vessel when that vessel visits a spoolbase.

In any technique for laying rigid pipe, it will be clear that delays infabricating the pipeline and applying field joint coatings will tie up avaluable capital asset in the form of an installation vessel that may beworth hundreds of millions of US dollars. Delays also increaseoperational costs of the vessel that may accumulate at a rate ofhundreds of thousands of US dollars per day. Delays also risk missing aweather window during which the pipeline can be laid in a satisfactorysea state, which could delay the entire subsea installation project ateven greater expense.

As delays may arise while waiting for chemical curing of a thermosetmaterial or for physical cooling of a thermoplastic material to solidifya field joint coating, various prior art proposals present solutions toquicken this hardening step. For example, one of the measures proposedin the aforementioned WO 2012/004665 is to place an insert into the gapbetween the parent coatings of abutting pipe joints before injectingmolten thermoplastics material into a mould placed around that gap,hence to embed the insert. The insert is a pre-fabricated shell orassembly of thermally insulating material, which may be applied to thepipeline offline as soon as the butt weld of that field joint has beentested. The insert reduces the volume of molten thermoplastics materialto inject, mould or cast and hence to cool down, thus reducing injectionand cool-down time. This provides a substantial gain in terms of cycletime and helpfully reduces the footprint of the equipment. It alsoimproves mechanical properties because internal stresses and strainsrelated to material shrinkage following injection or casting can bereduced significantly.

It will be apparent that whether S-lay, J-lay or reel-lay methods areemployed to lay a rigid pipe, the pipeline—including each of itssuccessive field joint coatings—will experience substantial stresses andstrains. Stresses and strains are experienced after a pipeline is laid,for example due to thermal cycling in use. However, stresses and strainsare particularly prevalent before and during laying as the pipeline isdeflected onto a reel, over an overbend or through a sag bend, as thecase may be, during spooling or laying. The stresses and strains aremost severe when spooling a coated pipeline onto a reel, which asmentioned above involves plastic deformation of the steel of the rigidpipe. The reel acting as a bending mandrel also imparts concentrateddeformation forces directly to the coating that act through the coatingon the underlying steel pipe.

When a pipeline undergoes substantial bending, cracks will tend toappear and de-bonding will tend to occur at the interfaces between fieldjoint coatings and parent coatings. The presence of an insert addsfurther interfaces and may increase the number of local stress andstrain concentrations within the field joint coating, which increasesthe risk of cracks appearing. Any such cracks may allow water to reachthe outer surface of the steel pipe, thus corroding the pipe. Wateringress may also reduce the adhesion of PU coatings to the pipe and mayadditionally degrade the coatings themselves. An example of suchdegradation is hydrolysis of a PU field joint coating under heatemanating from within the pipeline in use, which is particularlysignificant under the high-pressure conditions of deep water.Degradation or loss of adhesion of the coatings will tend to lead to afailure of thermal insulation and to permit further corrosion of thepipe.

The insert proposed in WO 2012/004665 requires injection of additionalmaterial to fill in the gaps left after the insert has been positioned.Filling in the injected material below the insert may be difficult toachieve. This challenge is addressed by providing channels or boresthrough, around or under the insert so that molten PP can flow from theouter gap between the mould wall and the insert to the inner gap betweenthe insert and the pipe wall. However this complicates the shape of theinsert and increases moulding or injection time. The insert designrequires careful attention to allow the injected PP to flow around theinsert, as flow paths can be limited and re-welding lines must beavoided in case they create points of weakness in the field jointcoating. Also, any cavities left in a field joint coating willconcentrate stress and may lead to failure of the coating in subseaservice, noting that such coatings typically have to withstand enormoushydrostatic pressures after installation of a pipeline at depth.

When overmoulding an insert as in WO 2012/004665, another challenge isto centre the insert in the mould cavity between the pipe and the mouldwall. In practice, the mould may need to be equipped with retractablepins to maintain gaps under and over the insert. Those gaps ensure thatthe molten PP will fill the cavity and bond to the pipe, the insert andthe parent coatings, hence ensuring continuity of thermal insulationacross the field joint.

WO 03/095887 also teaches pre-inserting and fastening solid shellelements around the field joint before injecting material. Again, achallenge here, is to achieve sufficient quality of injection below thesolid elements.

GB 2520717 describes placing solid thermoplastic shell elements around afield joint before heating the shell elements within a mould cavity.Thermal expansion of the shell elements is constrained within the mouldcavity during heating in order to apply elevated pressure between theshell elements and the pipeline.

Insulating or protective half-shells of plastics for pipelines aredisclosed in EP 2302277, for example. Such half-shells may be known inthe art as doghouses. Conventional protective half-shells such as theseare not designed to ensure a perfect interface with the pipes insidethem. Consequently, they leave voids at the interfaces, which rendersthem unsuitable for use in field joint coatings for subsea pipelines.

U.S. Pat. No. 8,506,735 describes coating a pipe using half-shell-shapedinsulation blankets that are covered by a protective jacket afterinstallation. However this is not relevant to the production of thermalinsulation coatings, especially field joint coatings, because of thegaps and voids that remain after installation.

WO 2012/172451 describes a method of applying protective polymersheeting to a cut-back on a pipeline by continuously extruding polymermaterial from a carriage as the carriage advances along an annular patharound the pipeline.

A field joint coating comprising successive layers or sleeves is knownfrom U.S. Pat. No. 3,744,823, which discloses a pipeline for carryingvery hot fluids such as molten sulphur. Again, the use of such astructure for subsea pipelines is precluded because gaps and voids arelikely to exist between the layers.

The present invention seeks to reduce the time required to produce arobust field joint coating that will resist cracking during reeling,installation and use of the pipeline.

Against this background, the invention resides in a method of protectinga field joint of a pipeline at which chamfered edges ofthermally-insulating parent coatings on conjoined pipe lengths are inmutual opposition about a longitudinally-extending gap. The method ofthe invention comprises: manufacturing an hourglass-shaped inner layeraround the pipe lengths, which layer extends longitudinally along thegap between the chamfered edges and at least partially overlies thechamfered edges; assembling a thermally-insulating solid insert from twoor more parts to lie in the gap surrounding the inner layer; applyingradially-inward pressure from the insert to the inner layer; andmanufacturing an outer layer around the insert using molten material toform a watertight barrier and to form one or more melted interfaces withthe inner layer.

The inner layer may be elastically compressed or plastically deformed bysaid radially-inward pressure. For example, where the inner layer is athermoplastic, the insert may be pressed into the inner layer when theinner layer is at a softening temperature.

The radially-inward pressure on the inner layer is preferably maintainedduring and after manufacturing the outer layer. This may be achieved byholding together the parts of the insert under tension in one or morefastenings that connect those parts. For example, the parts of theinsert may be held together by one or more straps under tension, by oneor more clips under resilient tension, or by engagement of barbs of oneof those parts with holes of another of those parts, that engagementmaintaining resilient deflection of the parts to keep the barbs undertension.

Conveniently, the parts of the insert may be press-fitted together toengage the fastenings and to apply radially inward pressure to the innerlayer. For example barbs may be an interference force-fit within theholes.

An anti-corrosion layer and an adhesive primer layer are suitablyapplied onto the pipe lengths before manufacturing the inner layer.

The inner layer is preferably moulded around the joined pipe lengths butcould instead be wound or wrapped around the joined pipe lengths.Similarly, the inner layer is preferably moulded around the insert butcould instead be wrapped around the insert For example, the inner layeror the outer layer could be extruded in situ.

The inventive concept also finds expression in a field joint arrangementfor a pipeline. That arrangement comprises: pipe lengths joinedend-to-end at a joint; thermally-insulating parent coatings on each ofthe pipe lengths, the parent coatings having chamfered edges that arespaced from the joint in mutual opposition about alongitudinally-extending gap; and an annular field joint coating. Inaccordance with the invention, that coating comprises: anhourglass-shaped inner layer that surrounds the pipe lengths, extendslongitudinally along the gap between the chamfered edges and hasinclined end parts that at least partially overlie the respectivechamfered edges; a thermally-insulating solid insert disposed in thegap, the insert being made up of two or more parts that cooperate tosurround the inner layer; and an outer layer surrounding the insert thatforms a watertight barrier and has one or more melted interfaces withthe inner layer.

To reduce stress in the field joint during bending, the insert ispreferably of a softer material than the inner layer.

The insert and the inner layer suitably cooperate to fill thelongitudinally-extending gap between the chamfered edges of the parentcoatings. For this purpose, the insert may have end faces inclined tolie closely against the end parts of the inner layer. Alternatively,there may be gaps between end faces of the insert and the end parts ofthe inner layer, which gaps are filled by material of the outer layer.

The end parts of the inner layer may extend longitudinally beyond theouter layer, which seals against the end parts of the inner layer atmelted interfaces. Preferably, however, the outer layer extendslongitudinally beyond the end parts of the inner layer to bond to theparent coatings outboard of melted interfaces with the end parts of theinner layer. Nevertheless, it is possible for the outer layer to bond tothe chamfers of the parent coatings.

The end parts of the inner layer may extend longitudinally beyond thechamfered edges to bond to radially outer surfaces of the parentcoatings. Alternatively, the end parts of the inner layer may terminateon the chamfered edges, radially inwardly with respect to radially outersurfaces of the parent coatings.

The end parts of the inner layer may terminate radially inwardly withrespect to a radially outer surface of the insert. Alternatively, theend parts of the inner layer may extend radially outwardly beyond aradially outer surface of the insert.

A radially outer surface of the insert suitably lies at substantiallythe same radius as radially outer surfaces of the parent coatings.

It is possible for the end parts of the inner layer to terminateradially at substantially the same radius as a radially outer surface ofthe outer layer.

The chamfered edges may each comprise at least one step coinciding witha boundary between layers of the parent coatings. For example, an innerlayer of each parent coating may protrude longitudinally into the gapfrom the chamfered edge. It is also possible for the end parts of theinner layer to extend into spaces defined by cutting back an outer layerof each parent coating from the chamfered edge.

To facilitate bending, the insert may comprise a longitudinal series ofsegments alternating with relatively flexible links. For example, thelinks may be relatively thin parts of the insert leaving gaps betweenthe segments on a radially outer side of the links, which gaps arefilled by material of the outer layer.

Preferably, the inner and the outer layers each have a thickness ofbetween one sixth and one quarter of the aggregate thickness of anassembly comprising the inner layer, the insert and the outer layer. Itis also preferred that the insert has an overall thickness of betweenone half and two thirds of the aggregate thickness of that assembly.

The inventive concept also encompasses a subsea pipeline comprising atleast one field joint arrangement of the invention.

In specific embodiments, the invention provides a thermally-insulatingfield joint coating that comprises, in radially outward succession:

-   -   a primer layer, which is typically of an adhesive on a        corrosion-resistant coating of FBE;    -   an hourglass-shaped thermally-insulating inner coating layer;    -   a solid insert comprising at least two cooperating        thermally-insulating shells; and    -   a watertight outer cover layer.

The hourglass-shaped layer may comprise a tape, a wrap or a sleeve, ormay be moulded in place, for example by injection moulding. Thehourglass-shaped layer may, for example, be between 10 mm and 25 mmthick and may be of any suitable coating material, even possibly FBEalthough preferably of moulded plastics.

The solid insert preferably compresses the hourglass-shaped layer. Thehourglass-shaped layer may be softer than the insert upon assembly, forexample by being heated to a softening temperature at that time.Preferably, however, the material of the insert is softer than thematerial of the hourglass-shaped layer at the temperatures thattypically prevail during spooling, straightening or installation. Thisreduces stress in the field joint coating as the underlying pipelinebends.

The insert and the hourglass-shaped layer suitably cooperate so astotally to fill the volume of the gap defined by cut-backs of the parentcoatings.

Shells of the insert suitably comprise at least one positioningprovision and at least one locking pin. The locking pin may engage abore in a cooperating shell of the insert, and could comprise a ratchet.Shells of the insert may be strapped together.

The outer cover layer is preferably of moulded polymer but could be atape, wrap or sleeve.

In a specific example, the invention provides a method for manufacturinga field joint coating, comprising:

-   -   preparing the field joint for coating, after welding;    -   applying a primer layer such as that described above;    -   manufacturing an hourglass-shaped layer that covers all of the        pipe surfaces of the field joint region between the chamfers of        the parent coatings and that extends to cover the chamfers, at        least partially;    -   positioning at least two cooperating thermally-insulating insert        shells around the hourglass-shaped layer;    -   pressing the shells against each other and/or against the        hourglass-shaped layer;    -   engaging at least one permanent locking device between the        shells;    -   releasing inward pressure exerted on the shells; and    -   manufacturing a watertight cover layer around the shells and the        ends of the parent coatings.

The invention combines partial moulding of a field joint coating,preferably involving IMPP, with the addition of a solid but potentiallystill flexible insert. The insert is then covered with a watertightouter layer.

The thickness of the first, hourglass-shaped inner layer is a trade-offbetween cooling time and the efficiency of thermal insulation. Thethinner the inner layer, the better to minimise cooling time. However, athicker inner layer is better for thermal insulation. A typicaloptimised value for the thickness of the inner layer is one sixth to onequarter of the overall thickness of the parent coating. For example, thethickness of the inner layer may be about 15 mm to 25 mm if thethickness of the parent coating is between 75 mm and 100 mm.

The inner layer is preferably moulded to optimise bonding with bothparent coatings and with the underlying pipe joints, although a tapecould be used to make the inner layer instead.

Assembling part-tubular parts of the insert such as half-shells aroundthe field joint creates a radially-inward pressure that compresses theinner hourglass layer. This inward pressure eliminates gaps between theinner layer and the underlying pipe joints and between the inner layerand the insert. Assembly and application of insert parts such ashalf-shells could be automated.

An outer layer such as a jacket or sleeve is required to ensurecontinuity of the global coating system, comprising the parent coatingand the field joint coating, that protects the pipe from sea water. Thatlayer is preferably moulded to optimise bonding with both parentcoatings, the insert and the inner layer, although again, a tape or awrap could be considered for the outer layer instead.

In a first step, a field joint is coated with a simple hourglass-shapedinjection-moulded inner layer of PP with a thickness of about 10 mm toabout 25 mm maximum, this being about one sixth to one quarter of theoverall thickness of the full field joint coating.

The next step, immediately after moulding the inner layer, is to fit theinsert at the same workstation or at the next workstation. Here, the PPof the hourglass-shaped insert is still soft but no longer molten. Forthis purpose, part-tubular insert parts such as two half-shells areassembled around, and pressed radially inwardly into, the inner layer,which flows plastically to conform closely to the inner side of theinsert. The thickness of the insert is around one half to two thirds ofthe overall thickness of the full field joint coating.

The final step is an overcoating of solid PP as an outer layer aroundthe insert to a thickness about one sixth to one quarter of the overallthickness of the field joint coating, typically about 10 mm to 15 mm.This preferably involves overmoulding of molten PP in a mould underpressure. It may instead be possible to apply a side wrap melted PP filmaround the insert with equivalent total thickness.

The invention allows the same mould tool to be used for mainline andtie-in field joints. Previously, mainline field joints have been madewith a thick layer of IMPP and tie-in field joints have been made with athinner hourglass-shaped layer of IMPP. In contrast, the same hourglassmould tool can be used for both mainline and tie-in field joints,reducing the overall cost.

By virtue of the invention, the insert can be simpler in shape than inthe prior art and does not require a complex mould with retractablepins, as there is no possibility of the insert moving within the mouldcavity. Part-tubular insert parts such as two half-shells can simply bepositioned over and pressed against the still-soft mouldedhourglass-shaped inner layer in close contact, thus achieving a properbond between the insert and the hourglass-shaped moulding.

Easy positioning and adjustment of the insert relative to thehourglass-shaped inner layer is achieved by virtue of the geometry ofthe outer surface of the hourglass shape and the precision allowed bythe IMPP process, which can achieve less than 0.1 mm tolerance. Theinsert can be injection moulded or machined with correspondingprecision.

The simple application of the half shells and their engagement with thehourglass-shaped inner layer allows intimately close contact betweenthem. There is useful contact pressure over the whole contact surfacesof the radially inner surface of the insert and the radially outersurface of the inner layer.

Manufacture of the inserts or their part-tubular parts can be performedin parallel before field joint fabrication. Depending on the thicknessof the inserts, they can be made by injection moulding in one or moresteps. For example, if the thickness of an insert is greater than about30 mm, a preform of 50% to 60% of the total thickness required can bemoulded and then the remaining volume of the insert can be overmouldedon the preform to reduce shrinkage and to improve manufacturingtolerances. A surface profile or texture can be applied to the insert toimprove mechanical bonding with the inner hourglass-shaped layer underthe insert and with the outer layer produced by overmoulding or appliedby tape or side wrap PP on top of the insert.

In order that the invention may be more readily understood, referencewill now be made, by way of example, to the accompanying drawings inwhich:

FIG. 1 is a schematic sectional side view of a coated field joint of apipeline as known in the prior art;

FIG. 2 is a flow diagram that expresses a prior art method of producingthe field joint coating of FIG. 1;

FIGS. 3a to 3j are a sequence of schematic side views that illustratethe execution of method steps for producing a field joint coating of theinvention, including detail views of the application of an insert to anunderlying inner layer of the field joint coating;

FIGS. 4 and 5 are flow diagrams that express methods of producing afield joint coating in accordance with the invention, as illustrated inFIGS. 3a to 3 j;

FIG. 6 is an enlarged schematic perspective view of an end of ahalf-shell being part of an insert in accordance with the invention;

FIG. 7 is a schematic sectional detail view of two of the half-shells ofFIG. 6 being assembled to make an insert;

FIG. 8 is a schematic perspective view of an insert in accordance withthe invention made from two of the half-shells shown in FIGS. 6 and 7;

FIG. 9 is a schematic side view of an inner layer of a field jointcoating being formed by winding a tape around a field joint;

FIG. 10 is a schematic side view of an outer layer of a field jointcoating being formed by side-wrapping an extruded wrap around a fieldjoint;

FIGS. 11a to 11c are a sequence of schematic side views that illustratethe execution of method steps for producing an alternative field jointcoating of the invention;

FIGS. 12 and 13 are schematic side views showing alternative approachesfor securing an insert around a field joint;

FIGS. 14 to 21 are schematic enlarged detail side views showing variousrelative dispositions of the inner layer, the insert and the outer layerof field joint coatings of the invention; and

FIGS. 22a and 22b are schematic enlarged detail side views of a furthervariant of the invention, in which the insert is adapted for increasedlongitudinal flexibility.

In the prior art arrangement shown in FIG. 1, a field joint is createdbetween abutting pipe joints 10 of a pipeline, where a circumferentialbutt weld 12 attaches the pipe joints 10 to each other end-to-end. Thepipe joints 10 could be double, triple or quadruple pipe jointsmanufactured in multiples of the standard length of about 12 m.Similarly, one or both of the pipe joints 10 could be at an end of amuch longer pipe stalk comprising numerous pipe joints 10 joinedend-to-end.

Each pipe joint 10 is coated with an insulating parent coating 14, forexample a SLPP coating, which terminates short of the facing end of eachpipe joint 10 with a typically chamfered end shape as shown. Thethickness of the parent coatings 14 is somewhat exaggerated in thisschematic view for clarity.

An annular gap lies between the opposed chamfered ends of the parentcoatings 14 around the weld 12. The exposed external surfaces of thepipe joints 10 are coated with an insulating field joint coating 16 thatfills the gap and substantially matches the radial thickness of theadjacent parent coatings 14.

In this description, references to the radial direction are defined withrespect to the common central longitudinal axis 18 of the abutting pipejoints 10, which is also the centre of curvature of the pipe joints 10,the coatings 14, 16 and other tubular or part-tubular features.

As acknowledged in the introduction, the field joint coating 16 is aptto be made using a mould tool fixed around the field joint. The mouldtool extends from one parent coating 14 to the other parent coating 14and overlaps them. This defines a mould cavity that includes the annulargap between the coatings 14 and that surrounds the field joint. A liquidpolymer such as PP is injected or otherwise introduced into the mouldcavity to harden in the mould cavity before the mould tool is removed tocoat another field joint of the pipeline. Mould tools suitable forproducing a field joint coating 16 are described in more detail in ouraforementioned International patent application published as WO2012/004665.

Injection-moulding of thermoplastics is preferred in the prior art andfor the purposes of the invention—especially when combined with theteachings of WO 2012/004665, such as sequential cascade injection.However, the invention is not confined to that possibility. For example,cast moulding of a thermoset field joint coating such as PU is alsopossible.

FIG. 2 is a flow diagram that expresses the prior art method ofproducing the field joint coating of FIG. 1. FIG. 2 illustrates thatmethod in the context of successive workstations (abbreviated here as‘WS’), which may be spaced from each other by the length of a pipe joint10 along the firing line of a spoolbase or of a pipelay vesselconfigured for S-lay operations.

To save time, different operations take place simultaneously onsuccessive field joints of a pipeline at the various workstations. Thepipeline is advanced in stepwise fashion along the firing line from oneworkstation to the next as soon as all operations have been completed atthe respective workstations. Consequently, all of those operations lieon the critical path, meaning that any of those operations has thepotential to delay the entire pipeline fabrication, installation orspooling process if that operation takes too long to complete.

FIG. 2 abbreviates the firing line to focus upon the operations that aremost relevant to field joint coating. Thus, various preceding operationssuch as bevelling and alignment of pipe joints, weld preparation,welding and NDT of the weld, which are typically performed at up to sixworkstations, are designated together in FIG. 2 as ‘Previous WS’.

After NDT of the weld, the field joint is moved to WS 7. There,preparation for field joint coating is performed by protecting thechamfered ends of the parent coatings 14 and then blasting the exposedexternal surfaces of the abutting pipe joints 10.

The pipeline is then advanced to bring the field joint to WS 8, wherethe exposed ends of the abutting pipe joints 10 are heated, for exampleby induction heating, and a layer of adhesive such as fusion-bondedepoxy (FBE) is applied to them as a primer.

When the field joint is advanced to WS 9, the chamfered ends of theparent coatings 14 are heated by radiant infra-red heaters to softenthem and the field joint coating (FJC) is produced frominjection-moulded PP (IMPP).

At WS 10, an optional protective finishing layer is applied around thefield joint coating by, for example, painting, overmoulding or applyinga tape or sleeve.

Finally, quenching may take place at WS 11 to cool and solidify thefield joint coating quickly. The field joint is then ready for furthersteps. If the firing line is on a pipelay vessel, those steps mayinvolve being launched with the adjoining pipe joints 10 into the sea.If the firing line is at a spoolbase, those steps may involve spoolingonto a reel.

Moving on now to field joint coating techniques of the invention,reference is made firstly to FIGS. 3a to 5 of the drawings.

FIGS. 3a to 3j illustrate the execution of some, but not all, of themethod steps of the invention. Like numerals are used for like parts.Whilst method steps that are not illustrated in FIGS. 3a to 3j will befamiliar to those skilled in the art, they are nevertheless describedbriefly below for ease of understanding.

FIGS. 4 and 5 are flow diagrams that correspond to FIG. 2 but showvariants of the method of the invention exemplified by FIGS. 3a to 3j .In those variants, method steps are performed in the same order asdescribed with reference to FIGS. 3a to 3j but some of those steps areperformed at different workstations, as will be explained.

FIG. 3a shows a field joint 20 created by a circumferential weld 12between abutting pipe joints 10 of a pipeline. As in FIG. 1, each pipejoint 10 is coated with an insulating parent coating 14, for example aSLPP coating. Each parent coating 14 terminates with a frusto-conicaledge chamfer 22 where the parent coating 14 is cut back from the weldedend of the associated pipe joint 10. This leaves exposed externalsurfaces 24 of the abutting pipe joints 10, ready for preparation byblasting after NDT has been performed on the weld 12 and the edgechamfers 22 have been suitably protected from the blasting process. Allof the aforementioned features are rotationally symmetrical about thecentral longitudinal axis 18.

FIG. 5 shows that preparation, chamfer protection and blasting may beperformed at WS 7, as in the prior art acknowledged in FIG. 2. However,FIG. 4 shows the alternative of deferring blasting until WS 8, if suchdeferral optimises the critical path.

FIGS. 4 and 5 show that the exposed external surfaces 24 of the pipejoints 10 are heated at WS 8, for example by induction heating or gasflame heating, and a layer of adhesive such as fusion-bonded epoxy (FBE)is applied to those surfaces 24 as a primer. Optionally, this isfollowed by infra-red heating of the edge chamfers 22 at WS 9. Next, amould tool 26 shown in FIG. 3b is assembled around the heated fieldjoint 20 from part-tubular mould parts 28. The mould parts are clampedtogether to hold the mould tool 26 in sealing engagement with the parentcoatings 14 of the pipe joints 10 and to resist internal pressure withina mould cavity 30 defined by the mould tool 26.

FIG. 3b shows that the mould tool 26 comprises a tube of generallycircular cross-section assembled around the field joint of FIG. 3a . Inthis example, the tube is divided longitudinally on a diameter of thecross-section into two halves 28 that seal against each other andagainst the parent coatings 14.

Opposed tubular end portions 32 of the mould tool 26 seat onto the outersurface of the parent coatings 14 and so have an internal diameter thatsubstantially corresponds to the external diameter of the parentcoatings 14. Conversely, a tubular central portion 34 between the endportions of the mould tool 26 has a relatively thick wall and acorrespondingly small internal diameter, which is smaller than theexternal diameter of the parent coatings 14. Frusto-conical steps 36 ateach end of the central portion extend between the central portion 34and the end portions 32, with an inclination to the longitudinal axisthat substantially matches that of the edge chamfers 22.

The internal diameter of the central portion 34 of the mould tool 26 isslightly greater than the external diameter of the exposed surfaces 24of the pipe joints 10. This spaces the central portion of the mould tool26 radially outwardly from the exposed external surfaces 24 of the pipejoints 10 to define the annular mould cavity 30 that encircles thoseexternal surfaces 24. The frusto-conical steps 36 are similarly spacedlongitudinally inwardly from the respective edge chamfers 22. Thus, themould cavity 30 also extends along the edge chamfers 22. The result isthat the mould cavity 30 comprises flared frusto-conical end spacesextending parallel to the edge chamfers 22, at respective ends of acentral tubular space that extends parallel to the external surfaces 24of the pipe joints 10.

The mould tool 26 carries internal seals 38 between the central portion34 and the respective end portions 32, at the radially outer ends of thefrusto-conical steps 36. The seals 38 encircle the pipe joints 10 todefine the longitudinal and radial extremities of the mould cavity 30.In this example, the seals 38 seal against the edge chamfers 22 of theparent coatings 14, near the radially outer edge of each edge chamfer22. Thus, the mould cavity 30 extends partially along the edge chamfers22, extending radially outwardly of the exposed external surfaces 24 ofthe pipe joints 10 but terminating radially inwardly of the outersurfaces of the parent coatings 14.

The tubular wall of the mould tool 26 is penetrated by one or more gates40 for injection of a liquid polymer into the mould cavity 30, in thisexample molten PP, supplied through feed lines 42 under pressure from asupplying reservoir or machine 44.

FIG. 3c shows an injection-moulded inner layer 48 of a field jointcoating of the invention, immediately after removal of the mould tool 26shown in FIG. 3b . In this example, the inner layer 48 is of solid PP.

The inner layer corresponds to the shape and extent of the mould cavity30 and so has a waisted shape known in the art as an ‘hourglass’. Thisterm is apt to describe a shape that comprises a slim or narrow waist50, midsection or joining segment disposed between relatively wide endcones 52, all being rotationally symmetrical about the centrallongitudinal axis 18. The term is particularly apt where the end cones52 have a taper whose inclination corresponds to the frusto-conicalshape of the edge chamfers 22. Thus, the wider end cones 52 of thehourglass shape extend part-way along the edge chamfers 22 whereas thenarrow waist 50 of the hourglass shape extends along the exposedexternal surfaces 24 of the pipe joints 10.

Moving on next to FIGS. 3d and 3e , these show, schematically, a tubularinsulating insert 54 in accordance with the invention being assembledaround the inner layer 48. The insert 54 is preferably of substantiallysolid thermally-insulating material such as GSPP, solid PP, glasssyntactic PU (GSPU) or solid PU. Whilst solid and substantially rigid,the insert 54 has sufficient inherent flexibility to bend with theunderlying pipe joints 10 as the pipeline bends during reeling orinstallation and in use.

The insert 54 is fused with the inner layer 48 under radially-inwardpressure, optionally with application of heat to the interface to meltor soften the materials. For example, parts of the insert 54 could bepre-heated before assembly around the inner layer 48.

The insert 54 is exemplified in FIGS. 3d and 3e as being dividedlongitudinally on a diameter of its cross-section into two half-shells56 that can abut each other along mutually-opposed side faces 58. Eachhalf-shell 56 is shaped to extend around half of the circumference ofthe inner layer 48, such that the half-shells 56 form a hollow cylinderor tube when they are brought together around the inner layer 48. Thus,a combination of two opposed abutting half-shells 56 forms a tube thatencircles the inner layer 48 which, in turn, surrounds the field joint20 as shown in FIG. 3e . Another example of such an insert is shownschematically in FIGS. 6 to 8, which will be described later.

FIG. 3d shows the half-shells 56 being brought together around the fieldjoint 20 in longitudinal alignment with the annular gap between theopposed edge chamfers 22. One of the half-shells 56, which is uppermostin the drawings, is drawn in longitudinal section to show the internalshape of the half-shells 56. Conversely, FIG. 3d shows only the outerside of the other half-shell 56.

In FIG. 3e , the half-shells 56 have been brought together around thefield joint 20 to come together along their opposed side faces 58 andhence to form the tubular insert 54. The insert 54 bridges the remainderof the gap between the opposed edge chamfers 22. There should beintimately close contact between the radially inner side of the insert54 and the radially outer side of the inner layer 48. The radially innerside of the insert 54 is therefore shaped like the inner side of themould tool 26 so as to match the radially outer side of the inner layer48 that was created by the mould tool 26.

Specifically, the geometry of the insert 54 fits into a tubular volumein the shape of a thick-walled hollow cylinder. The insert 54 has aradially outer face 60 whose external diameter is preferably slightlyless than, or substantially equal to, the external diameter of theparent coatings 14. In the example shown, the outer face 60 of theinsert 54 lies radially within the radial extremity of the inner layer48, as defined by the end cones 52 of the inner layer 48 that protrudelongitudinally and radially from respective ends of the insert 54 asshown in FIG. 3 e.

The insert 54 also has a radially inner face 62 whose internal diameteris substantially equal to, or slightly smaller than, the externaldiameter of the inner layer 48. Opposed ends 64 of the insert 54 have ahollowed frusto-conical concave profile to correspond to the opposedconvex contour of the end cones 52 of the hourglass-shaped inner layer48 that overlie the edge chamfers 22 of the parent coatings 14.

FIGS. 3d and 3e show the two half-shells 56 of the insert 54 beingforced together by radially-inward pressure. In this example, thatpressure is exerted by a clamping apparatus 66 comprising double-actinghydraulic or pneumatic actuators 68 bearing on diametrically-opposedradially-movable reciprocating jaws 70. Extending the actuators 68 in anassembly stroke as shown in FIG. 3e moves the jaws 70 toward each otherand hence forces together the two half-shells 56 of the insert 54between the jaws 70. In this respect, FIG. 4 shows application of thehalf-shells 56 as the final operation at WS 9 whereas FIG. 5 showsapplication of the half-shells 56 as the first operation at WS 10.

When the actuators 68 retract in a return stroke, they pull the jaws 70away from the assembled insert 54, allowing the insert 54 subsequentlyto be carried downstream by stepwise progress of the pipeline along thefiring line. The jaws 70 are then loaded with fresh half-shells 56 readyfor the assembly stroke to begin again, when assembling a further insert54 around the next field joint 20 in the upstream direction. Thus, thenext insert 54 can be assembled at the same workstation from furtherhalf-shells 56, upstream of the field joint 20 that carries thepreceding insert 54.

Preferably, when assembling and applying the insert 54 as shown in FIGS.3d and 3e , the inner layer 48 of PP remains above a softeningtemperature at which application of external pressure to the inner layer48 can cause the PP to flow plastically. The optimum temperature is oneat which the inner layer 48 has cooled to achieve self-supportingrigidity or viscosity but remains susceptible to plastic deformationunder radially inward pressure applied via the insert 54. An exemplarytemperature range in this respect is between 125° C. and 145° C. Thiscan be achieved by removing the mould tool 26 before the inner layer 48has cooled fully and then immediately forcing together the half-shells56 of the insert 54 around the still-hot inner layer 48. In thisrespect, FIG. 4 shows that the half-shells 56 of the insert 54 may beapplied when the field joint 20 is at WS 9, immediately after IMPPmoulding of the hourglass-shaped inner layer 48. However, FIG. 5 showsthat the half-shells 56 of the insert 54 could instead be applied afteradvancing the field joint 20 to WS 10.

If needs be, and if critical path analysis allows, the inner layer 48can be reheated before applying the half-shells 56 of the insert 54, forexample by using an infra-red heater surrounding the field joint 20.

The consequence of pressing the half-shells 56 of the insert 54 into astill-soft inner layer 48 is shown in the sequence of enlarged detailviews in FIGS. 3f to 3h . FIG. 3f shows one of the half-shells 56 lyingloosely against the inner layer 48. FIG. 3g shows the half-shell 56 nowpressed by one of the jaws 70 against the inner layer 48, which at thisstage is not yet deformed by inward pressure exerted through thehalf-shell 56. FIG. 3h shows the half-shell 56 pressed further by thejaw 70 into the inner layer 48, which has flowed under the resultinginward pressure to deform plastically around the stiffer material of thehalf-shell 56.

As FIG. 3h shows, the result is that the contacting outer surface of theinner layer 48 conforms to the internal shape of the half-shell 56. Theresulting flow of PP slightly reduces the thickness of the inner layer48 between the end 64 of the half-shell 56 and the opposed edge chamfer22 and between the central tubular wall of the half-shell 56 and theexposed surfaces 24 of the pipe joints 10. PP of the inner layer 48displaced by the continuing inward movement of the half-shell 56 flowsaround the half-shell 56, which may therefore emboss or become partiallyembedded in the inner layer 48. This creates an intimately close fitbetween the half shell 56 and the inner layer 48, improving adhesion andmechanical engagement between them. The closeness of the fit reduces thepossibility of cracks developing at interfaces between the half shell 56and the inner layer 48 when the pipeline bends in use or duringinstallation, spooling or straightening.

Returning to FIGS. 3d and 3e , these drawings shows one way in whichpart-tubular sections of an insert 54 such as half-shells 56 can bejoined to each other to hold them together around a field joint 20. Inthis example of fastenings disposed on the half-shells 56, pins or barbs72 projecting tangentially from a side face 58 of a first half-shell 56are received in respective tangentially-extending holes or bores 74 inan opposed side face 58 of a second half-shell 56. Similar fasteningsmay be distributed along the length of the insert 54 and to both sidesof the insert 54.

Specifically, in this example, each side face 58 of a half-shell 56 hasan array of longitudinally-spaced barbs 72 that project orthogonallyfrom that side face 58 in positions to align with correspondingly-spacedbores 74 in an opposed side face of the other half-shell 56. The barbs72 alternate with bores 74 that are positioned to align withcorrespondingly-spaced barbs 72 on the opposed side face 58. Thearrangement of the barbs 72 and the bores 74 is such that when twohalf-shells 56 are aligned face-to-face for assembly to form the insert54, the barbs 72 of each half-shell 56 align with the bores 74 of theother half-shell 56. Distal ends of the barbs 72 on each side face 58 ofa half-shell 56 initially locate in the bores 74 in the counterpart sidefaces 58 of the opposed half-shell 56. Radially-inward pressure thenforces the half-shells 56 together as the barbs 72 are urged deeper intothe bores 74. The barbs 72 thus engage with the opposed bores 74 whenthe half-shells 56 are pressed together around the field joint 20 by thejaws 70.

Thus, the insert 54 is apt to be assembled in a simple process providingspeed, clamping strength and safety. The half-shells 56 are broughttogether as two halves from opposite sides of the field joint 20 and areassembled robustly in a simple press-fit operation with predictable andeasily-verifiable results. If desired, the process could be largelyautomated.

Slight resilience of the half-shells 56 helps to ensure a snug fitaround the inner layer 48. The resilience of the half-shells 56 alsoapplies a continuous clamping force to the pipe joints 10 via the innerlayer 48. This clamping force helps to avoid movement of the insert 54with respect to the field joint 20, whether axially along the pipejoints 10 or circumferentially around the pipe joints 10.

Moving on now to FIG. 3i , this shows a second mould tool 76 assembledaround the field joint 20, where the insert 54 has previously beenassembled around the inner coating 48 that overlies the pipe joints 10.The purpose of the second mould tool 76 is to overmould an outer layer78 as seen in FIG. 3 j.

FIGS. 4 and 5 show that overmoulding of the outer layer 78 is apt to beperformed at WS 10 before quenching at WS 11 if needs be. Optionally,the external surface of the insert 54 and the exposed parts of the innerlayer 48 and the edge chamfers 22 are heated, for example by infra-redheating, before overmoulding of the outer layer 78 takes place.

Again, the second mould tool 76 comprises a tube of generally circularcross-section formed of part-tubular mould parts that are clampedtogether. For example, as before, the tube is suitably dividedlongitudinally on a diameter of the cross-section into two halves 80.Clamping force between those halves 80 holds the second mould tool 76 insealing engagement with the parent coatings 14 of the pipe joints 10 andresists internal pressure within a mould cavity 82 defined by the secondmould tool 76.

Opposed tubular end portions 84 of the second mould tool 76 seat ontothe outer surface of the parent coatings 14 and so have an internaldiameter that substantially corresponds to the external diameter of theparent coatings 14. Conversely, a tubular central portion 86 between theend portions of the second mould tool 76 has a relatively thin wall anda correspondingly larger internal diameter that exceeds the externaldiameter of the parent coatings 14. This allows for contraction of theouter layer 78 after overmoulding.

The central portion 86 of the second mould tool 76 is spaced radiallyoutwardly from the external surface 88 of the insert 54 to define theannular mould cavity 82 encircling that external surface. The mouldcavity 82 also extends over the exposed end cones 52 of the inner layer48 and the edge chamfers 22 and slightly overlaps the external surfaceof the parent coatings 14. The second mould tool 76 carries internalseals 90 that encircle the pipe joints 10 and seal against the externalsurface of the parent coatings 14 to define the longitudinal extremitiesof the mould cavity 82.

Again, the tubular wall of the second mould tool 76 is penetrated by oneor more gates 92 for injection of a liquid polymer into the mould cavity82, in this example molten PP, supplied through feed lines 94 underpressure from a supplying reservoir or machine 96.

FIG. 3j shows the injection-moulded outer layer 78 of a field jointcoating of the invention, immediately after removal of the second mouldtool 76 shown in FIG. 3i . In this example, the outer layer 78 is ofsolid PP. The outer layer 78 corresponds to the shape and extent of themould cavity 82 defined by the second mould tool 76. The outer layer 78may take on a slightly narrowed, waisted shape as the PP contracts whenit cools.

Turning next to FIGS. 6 to 8, these drawings show possibleconfigurations of an insert 96 formed of part-tubular parts such as twohalf-shells 98.

FIG. 6 is a detail view that shows one of the half-shells 98 inisolation. The wall thickness of the half-shell 98 is exaggerated hereto show fastenings and interengagement formations that are provided onits side faces 100. On assembly of the insert from two such half-shells98, which are preferably identical, the side faces 100 of the half-shell98 shown in FIG. 6 will cooperate with opposed side faces 100 of theother half-shell 98 of the insert 96. The fastenings and interengagementformations of the side faces 100 of the half-shell 98 are thereforemirrored with respect to a plane that contains those side faces 100.

One of the side faces 100, shown to the left in FIG. 6, comprises aninner longitudinally-extending row of tangentially-projecting pins orspigots 102 and an outer longitudinally-extending row oftangentially-extending holes 104. A longitudinally-extending groove 106lies between those rows. The other of the side faces 100, shown to theright in FIG. 6, has a mirrored arrangement that comprises an outerlongitudinally-extending row of spigots 102 and an innerlongitudinally-extending row of holes 104. A longitudinally-extendingridge 108 lies between those rows.

As best appreciated in the sectional detail view of FIG. 7, the spigots102 are shaped, spaced, positioned and dimensioned to engage with theholes 104 of another identical half-shell 98 and vice-versa. Similarly,the ridge 108 is shaped, positioned and dimensioned to fit into thegroove 106 of another identical half-shell 98 and vice-versa. Thus, thespigots 102 and the holes 104 serve as fastenings and themutually-complementary ridge 108 and groove 106 serve as optionalinterengagement formations that locate the half-shells 98 with respectto each other upon assembly of the insert 96.

One end of the resulting insert 96 is shown schematically in FIG. 8 ofthe drawings, which shows the half-shells 98 abutting along theirmutually-opposed side faces 100. FIG. 8 also shows the hollowedfrusto-conical concave profile at the end of the insert 96, whichcorresponds to the opposed convex contour of the end cones 52 of theinner layer 48.

FIG. 9 shows an alternative approach for forming the inner layer 48.Here, an hourglass-shaped inner layer 48 is being formed fromoverlapping coils of heated helically-wound tape 110 wrapped around theedge chamfers 22 and the exposed surfaces 24 of the pipe joints 10. Asimilar approach can be used for the finishing operations that may beperformed at WS 10 in FIGS. 4 and 5.

FIG. 10 shows, schematically, an alternative approach for forming theouter layer 78. Again, a similar approach can be used for the finishingoperations that may be performed at WS 10 in FIGS. 4 and 5. In FIG. 10,a side-wrap coating apparatus 112 comprises a robot head 114 driven bymotors 116 along circumferential tracks 118 that are clamped to theouter surfaces of the parent coatings 14.

As it moves around the field joint along the tracks 118, the robot head114 extrudes and wraps a molten polymer film 120 around the insert 54 toform the outer layer 78. The outer layer 78 is shown here only partiallyformed because the robot head 114 has yet to complete a full circuit ofthe field joint.

The film 120 covers the insert 54 and overlaps the end cones 52 of theinner layer 48 and the edge chamfers 22 of the parent coatings 14 tobond to the outer surfaces of the parent coatings 14. The method andapparatus are described fully in WO 2008/132279.

FIGS. 11a to 11c show an alternative configuration for anhourglass-shaped inner layer 48, whether that layer is moulded inaccordance with FIGS. 3a to 3c or formed of tape in accordance with FIG.9. Here, the inner layer 48 does not terminate longitudinally part-wayalong the edge chamfers 22 but instead overlaps the edge chamfers 22,thus extending onto and overlapping the external surfaces of the parentcoatings 14. Otherwise, FIG. 11a corresponds to FIG. 3c , showing thefield joint 20 surrounded by the inner layer 48, before the insert 54 isplaced around the inner layer. FIG. 11b corresponds to FIG. 3d but omitsthe clamping apparatus, and shows the half-shells 56 of the insert 54being assembled around the field joint 20. FIG. 11c corresponds to FIG.3i , and shows a second mould tool 76 positioned around the assembly ofthe field joint 20 and the insert 54, ready to overmould an outer layer78.

It will be noted from FIG. 11c that the mould cavity 82 encompasses theend cones 54 of the inner layer 48 that overlap the external surfaces ofthe parent coatings 14. Thus, the outer layer 78 will extendlongitudinally outboard of the inner layer 48 in this example. However,it would be possible for the arrangement instead to be reversed so thatthe outer layer 78 terminates longitudinally inboard of the overlappingend cones 52 of the inner layer 54.

FIGS. 12 and 13 show some alternative approaches for holding togetherpart-tubular parts of an insert 54, again exemplified here ashalf-shells 56. FIG. 12 shows alternative fastenings comprising a key orlatch 122 that, by resilient engagement, an over-centre action or asnap-fit, engages the half-shells 56 and bridges the joint between them.In FIG. 13, tensile straps or bands 124 extend circumferentially aroundthe half-shells 56 and are tightened to pull the half-shells 56together.

FIGS. 14 to 21 show various options for the relative dispositions of theinner layer 48, the insert 54 and the outer layer 78 with respect to theedge chamfer 22 and the external surface of the adjacent parent coating14. For clarity, the inner layer 48 is not shown in these drawings asbeing deformed by radially-inward pressure exerted via the insert 54, asis shown in FIGS. 3f to 3 h.

FIGS. 14 to 17 have some features in common. In particular, the flaredend cone 52 of the hourglass-shaped inner layer 48 extends onlypartially along the edge chamfer 22. Thus, the end cone 52 extendsradially outwardly relative to the exposed external surface 24 of theunderlying pipe joint 10 but terminates radially inwardly relative tothe outer surface of the parent coating 14. Also, the outer layer 78extends longitudinally beyond the edge chamfer 22 to overlap onto theouter surface of the parent coating 14.

FIGS. 14 and 15 have a further feature in common, which is that thefrusto-conical end 64 of the insert 54 substantially matches theinclination of, and lies tightly against, the end cone 52 of the innerlayer 48 that lies, in turn, against the edge chamfer 22.

In FIG. 14, the radially outer side 88 of the insert 54 lies radiallyinwardly with respect to the radially outer edge of the end cone 52 ofthe inner layer 48. Thus, the end of the insert 54 extends onlypartially along the end cone 52. This arrangement is currently preferredbecause it reduces the number and length of interface surfaces betweenthe inner layer 48, the insert 54, the outer layer 78, the edge chamfer22 and the external surface of the parent coating 14.

In contrast, FIG. 15 shows the radially outer side 88 of the insert 54lying radially outwardly with respect to the radially outer edge of theend cone 52 of the inner layer 54. In this example, the radially outerside 88 of the insert 54 optionally lies on substantially the sameradius as the outer surface of the parent coating 14. Also, the end 64of the insert 54 extends fully along the end cone 52 of the inner layer48. This leaves a radially-overlapping portion of the insert 54 thatextends radially beyond the end cone 52 and that is spacedlongitudinally from the edge chamfer 22.

Optionally, the radially-overlapping portion of the insert 54 may bebevelled as shown in FIG. 15. This bevel 126 promotes flow of thematerial of the outer layer 78, during overmoulding, into the shallowgap 128 between the insert 54 and the edge chamfer 22, which gap 128 isdisposed radially outwardly from the end cone 52 of the inner layer. 48

FIGS. 16 and 17 have a different feature in common, which is that theend of the insert 54 is spaced from the end cone 52 of the inner layer48. This leaves a deep gap 130 between the end 64 of the insert 54 andthe end cone 52. The material of the outer layer 48 flows in to fillthat gap 130 during overmoulding.

Where there is a gap 130 between them, the inclination of the end 64 ofthe insert 54 does not have to match the inclination of the end cone 52of the inner layer 48. In the examples shown, the end 64 of the insert54 splays apart from the end cone 52 in the radially outward direction.The resulting radially-inward taper of the gap 130 eases inward flow ofthe material of the outer layer 78 into the gap 130 during overmoulding.Indeed, the inclination of the end 64 of the insert 54 could be reversedto face away from the end cone 52, maximising the ease of inward flow ofthe material of the outer layer 78.

The gap 130 between the end 64 of the insert 54 and the end cone 52 ofthe inner layer 48 can extend radially inwardly to any depth. In theseexamples, the gap 130 is deep enough to expose part of the centraltubular portion 50 of the inner layer 48.

In FIGS. 18 and 19, the end cone 52 of the inner layer 48 extends alongthe full length of the edge chamfer 22 and overlaps onto the outersurface of the parent coating 14. Thus, the end cone 52 extendslongitudinally beyond the edge chamfer 22. The outer layer 78 overlies,and extends longitudinally beyond, the overlapping part of the end cone52 to join the outer surface of the parent coating 14 at a positionlongitudinally outboard of the end cone 52. Optionally, as also shown,the radially outer side 88 of the insert 54 lies on substantially thesame radius as the outer surface of the parent coating 14.

In both of the examples shown in FIGS. 18 and 19, the end cone 52extends radially outwardly beyond the outer surface 88 of the insert 54.Whilst in principle it would be possible to have an opposite arrangementin which the outer surface 88 of the insert 54 extends radiallyoutwardly beyond the end cone 52, in practice the overlying outer layer78 could protrude too far radially in that case.

FIG. 18 shows the frusto-conical end 64 of the insert 54 substantiallymatching the inclination of, and lying tightly against, the end cone 52of the inner layer 48, like FIGS. 14 and 15. In contrast, like FIGS. 16and 17, FIG. 19 shows the end 64 of the insert 54 spaced from the endcone 52 to leave a deep gap 130 between the end of the insert 54 and theend cone 52, into which the material of the outer layer 78 flows duringovermoulding. Again, that gap 130 is shown here as extending radiallyinwardly to the extent that it exposes part of the central tubularportion 50 of the inner layer 48. Similarly, the inclination of the end64 of the insert 54 may differ from the inclination of the end cone 52,resulting in a gap 130 that tapers radially inwardly as shown, and theinclination of the end 64 of the insert 54 could be reversed to faceaway from the end cone 52.

A bevel 126 like that shown in FIG. 15 could be applied to any of theinserts 54 shown in FIGS. 16, 17 and 19, for the same purpose of easinginward flow of the material of the outer layer 78 into the gaps 130 ofthose embodiments.

FIGS. 20 and 21 show further possible variants of the invention in moredetail than FIGS. 14 to 19. In these drawings, the SLPP parent coating14 is shown in layered detail, comprising: an FBE primer layer 132 overthe pipe joint 10; an inner coating layer 134 that comprises a thinlayer of PP bonded to the primer and a thicker layer of extruded PPapplied over the thin bonded layer; an intermediate coating layer 136 ofPP modified for thermal insulation, exemplified here by GSPP or a foam;and an outer coating layer 138 of extruded PP. In this example,optionally, the edge chamfer 22 is stepped such that the inner coatinglayer 134 protrudes longitudinally into the annular gap thataccommodates the insert 54. This ensures better tightness. Optionally,as also shown in FIGS. 20 and 21, the radially outer side 88 of theinsert 54 lies on substantially the same radius as the outer surface ofthe parent coating 14.

In the variant of FIG. 20, the outer coating layer 138 has a chamferedface in alignment with the chamfered face of the intermediate coatinglayer 136. Conversely, the edge chamfer 22 in the variant of FIG. 21 isfurther stepped, in that the outer coating layer 138 is cut back, with achamfer, from the chamfered face of the intermediate coating layer 136.

In FIG. 20, the end cone 52 of the inner layer 48 extends along the fulllength of the edge chamfer 22 and overlaps onto the outer surface of theparent coating 14. Thus, the end cone 52 extends longitudinally beyondthe edge chamfer 22 onto the outer surface of the parent coating 14.This is similar to the arrangement shown in FIGS. 18 and 19. However,unlike the arrangement shown in FIGS. 18 and 19, the outer layer 78 doesnot overlie or extend longitudinally outboard beyond the overlappingpart of the end cone 52. Instead, that arrangement is reversed so thatthe outer layer 78 terminates against the longitudinally inboard side ofthe end cone 52. Beneficially for the overall thickness of the fieldjoint coating, this allows the outer layer 78 to be kept flush with, oreven to be kept radially within, the radial extremity of the end cone52.

The arrangement shown in FIG. 21 achieves a similarly beneficialreduction of the overall thickness of the field joint coating byrecessing an overlapping part of the end cone 52 into the space createdby cutting back the outer coating layer 138. The outer layer 78 can nowoverlie and extend longitudinally outboard beyond the overlapping partof the end cone 52 without adding to the thickness of the field jointcoating.

In general, it may be preferred that the outer layer 78 overlaps beyondthe edge chamfers 22, for example onto the outer surface of the parentcoating 14. This because a typical overlap of 50 mm to 75 mm allows alarge tolerance in the axial length of the gap between the edge chamfers22 that accommodates the insert 54.

Turning finally to FIGS. 22a and 22b , these drawings show how an insert140 may be adapted for increased longitudinal flexibility. Thisadaptation enables the insert 140 to bend more easily along its lengthas the underlying pipe joints 10 bend. Whilst shown in the context ofthe arrangement of the inner and outer layers 48, 78 as shown in FIG.18, a similar insert 140 could of course be used in any of the precedingembodiments.

The insert 140 shown in FIGS. 22a and 22b comprises a longitudinalseries of segments 142 connected by joints or links 144, such that thesegments 142 alternate with the links 144 along the length of the insert140. Broadly, the segments 142 and the links 144 of the insert 140together behave like a vertebral column or spine, in which the segmentsare akin to vertebrae and the links are akin to intervertebral discsinterleaved between the vertebrae.

In the example shown in FIGS. 22a and 22b , the segments 142 and thelinks 144 are annular discs or hoops that encircle the pipe joints 10and that are integral with each other to form a one-piece insert 140.Both sides of each segment 142 are flat and lie in parallel planes thatare orthogonal to the central longitudinal axis of the pipe joints 10.The dimensions and numbers of segments 142 and links 144 may be chosento suit the material used for the insert 140, the performancerequirements of the field joint coating and expected strains thatdetermine the requirement for resilience in the field joint and itscoating.

The segments 142 and the links 144 are flush on their radially innersides where the radially inner face of the insert 140 lies against theunderlying pipe joints 10. However, the links 144 are substantiallyshallower in the radial direction than the segments 142, thus defining acastellated longitudinal section on the radially outer face of theinsert 140. The resulting gaps 146 between adjacent segments 142 allowclearance for relative angular displacement between the segments 142 asthe pipeline and hence the insert 140 bends along its length.

By way of example, each segment 142 may be about 30 mm widelongitudinally and about 30 mm high radially, atop a radially-inwardcore tube 148 that is about 30 mm thick radially. Thus, the insert 142has a radial thickness of about 60 mm atop the hourglass-shaped innerlayer 48. In this example, the gaps 146 between two successive segments142 may be about 15 mm wide longitudinally and about 30 mm deepradially.

When an outer layer 78 is over-moulded by casting or injection aroundthe insert 140, the material of the outer layer 78 flows into and fillsthe gaps 146 between the segments 142 as shown in FIG. 22b . Thematerial of the outer layer 78 is flexible enough to accommodate angulardisplacement between the segments 142 when the insert 140 bends.

The segments 142 of the insert 140 are preferably of a substantiallysolid thermally-insulating material such as GSPP. The links 144 can bemade flexible relative to the segments 142 in various ways. Thesegmented, jointed arrangement of the insert 140 confers flexibility onthe insert 140 to bend along its length in response to correspondingbending of the pipeline. The readiness of the insert 140 to bend in thisway reduces stress in a field joint coating that incorporates the insert140. This reduces initiation and propagation of cracks in and betweenthe layers 48, 78 and the insert 140 of the field joint coating andbetween those layers 48, 78, the insert 140 and the adjacent parentcoatings 14.

The links 144 may be of the same material as the segments 142, as shownin FIGS. 22a and 22b , or may be of a different material. The links 144could be intrinsically flexible either by virtue of their material ortheir structure, cross-sectional shape or dimension. In this example,the links 144 and segments 142 are all integral with each other in aone-piece insert 140, which could be machined into its final shape froma moulded or cast block of material or moulded directly in that finalshape. Where they are of the same material as the segments 142, thelinks 144 may be integral with the segments 142 as shown, beingrelatively shallow or thin in the radial direction to confer greaterrelative flexibility on the links 144. Thus, the links 144 shown inFIGS. 22a and 22b are relatively thin webs that bridge the gaps 146 toconnect adjacent segments 142.

The insert 140 can bend without significantly affecting its ability toinsulate the pipeline. In this respect, it will be noted that theinsulating segments 142 extend to substantially the same radius as thethickness of the parent coating 14. Also, the segments 142 are able towithstand radially-inward compressive forces experienced by the pipelineduring spooling and installation and under hydrostatic pressure in use.

If of a different material to the segments 142, the links 144 may be ofa more flexible material than the segments 142. In that case, the links144 need not be shallower or thinner in the radial direction than thesegments 142, although they could be. Indeed, the links 144 may be asthick in the radial direction as the segments 142. For example, thelinks 144 could be of a resilient rubber or gel-like material that maybe interleaved between the segments 142 or moulded between the segments142.

The insert could instead be an assembly of elements comprising aplurality of segments and a plurality of links. Such elements may or maynot be of the same material. Each link could comprise two or moresubstantially rigid parts that are hinged, jointed or articulated toconfer flexibility on the link as a whole.

The outer layer could be replaced with insulating infill mouldings of aflexible insulating material that are shaped to fill the gaps betweensegments of an insert. The infill mouldings could be moulded separatelyfrom and assembled with the insert or may be moulded in situ around andbetween the segments of the insert, by placing the insert in a secondarymould for overmoulding with the infill material as described above.

Many other variations are possible within the inventive concept. Forexample, the pipeline may intermittently be held stationary or may movecontinuously along the firing line during assembly of the insert fromthe half-shells. Thus, the clamping apparatus shown schematically inFIGS. 3d and 3e may take other forms to allow for continuous movement ofthe pipeline. For example, the jaws may be supported by alongitudinally-reciprocating carriage that allows the half-shells to beengaged while the pipeline is moving along the firing line. Thus, duringan engagement stroke of the jaws, the carriage can move longitudinallyfrom a start position in the direction of movement of the pipeline asthe jaws also come together to assemble the insert. Once the half-shellsare fully engaged at the end of the engagement stroke, the jaws separateto free the insert and the carriage moves back to the start positionagainst the direction of movement of the pipeline, during a returnstroke of the jaws.

The barbs and the bores on the cooperable parts that make up an insertmay have various configurations or may be replaced with otherfastenings. For example, a barb may be replaced by a spigot with anenlarged head that snap-fits resiliently into an undercut recess in anopposed bore when the half-shells are pressed together. Alternatively,the cooperable parts of an insert may comprise complementary ratchetformations.

Half-shells or other part-tubular sections of an insert can be pressedradially inwardly into contact with the inner layer by a pressing memberother than a jaw, by a tensioning apparatus comprising ratchet straps,by a torque-gauged device or by external fluid pressure to promoteattachment of the insert to the inner layer. Once assembled, thepart-tubular sections of the insert can be held together and/or to theinner layer by adhesive, by mechanical engagement, by externalfastenings or by fusing, welding or other bonding.

To improve bonding or adhesion between the various components of thefield joint coating, the insert could be pre-coated or overmoulded witha skin of PP or of another polymer that is compatible with the innerlayer shown in FIG. 3c and/or with an outer layer shown in FIG. 3j . Theskin could cover the insert fully or only partially. The skin could besupplemented with, or replaced by, a layer of adhesive.

The half-shells may be of cast or injection-moulded plastics materialand the barbs may be of steel, although other materials are possible.Half-shells may be moulded around the barbs in an insert or outsertmoulding process or the barbs may be engaged in mounting holes providedin pre-moulded half-shells. There may, for example, be a threadedengagement between the barbs and the mounting holes. Alternatively,there may be an interference fit between the barbs and the mountingholes, whose strength may be increased by ribbing, threading orotherwise texturing a root portion of a barb to be received in amounting hole.

Many different profiles or textures such as ribbing, threading orknurling may be applied to the barbs to tailor insertion and withdrawalforces into and out of the bores. Various examples of such barb profilesare discussed in WO 2013/008021.

The half-shells may be joined by a pivot or hinge arrangement to closearound the field joint in a clamshell arrangement. In that case, thehalf-shells suitably pivot relative to each other about a longitudinalpivot axis extending parallel to the central longitudinal axis 18 of thepipe joints 10.

As is known from prior art such as WO 2012/004665, each gate of a mouldtool may have a respective valve that controls the injection of liquidpolymer through that gate. The valves may be controlled by a centralcontrol unit and may be operated independently of each other. These andother mould tool features have been omitted from FIGS. 3b and 3i forsimplicity, such as vents to allow air to escape as the mould cavityfills with liquid polymer, and external clamps. Also, if the liquidpolymer is of molten thermoplastics such as PP, a cooling system may beprovided to accelerate hardening of the melt. The cooling system could,for example, comprise a water jacket disposed in or on the tubular wallof the mould tool.

Additives or modifiers may be employed in the insert or the field jointcoating, such as an elastomeric modifier like EDPM (ethylene propylenediene monomer rubber) to provide appropriate flexibility and impactresistance, or fibres of glass, aramid or carbon to increase strengthand elastic modulus.

Thermoplastics material used for injection-moulding the insert or thefield joint coating may be PP, polystyrene or any other suitablethermoplastics material that is compatible with the coating applied tothe pipe joints. Additives such as fibres may reduce shrinkage andaccelerate cooling.

Those skilled in the art will appreciate that combinations of featuresof the embodiments disclosed above are possible, even if thosecombinations are not explicitly recited in the foregoing description.

1. A method of protecting a field joint of a pipeline at which chamferededges of thermally-insulating parent coatings on conjoined pipe lengthsare in mutual opposition about a longitudinally-extending gap, themethod comprising: manufacturing an hourglass-shaped inner layer aroundthe pipe lengths, which layer extends longitudinally along the gapbetween the chamfered edges and at least partially overlies thechamfered edges; assembling a thermally-insulating solid insert from twoor more parts to lie in the gap surrounding the inner layer; applyingradially-inward pressure from the insert to the inner layer; andmanufacturing an outer layer around the insert using molten material toform a watertight barrier and to form one or more melted interfaces withthe inner layer.
 2. The method of claim 1, comprising elasticallycompressing the inner layer by said radially-inward pressure.
 3. Themethod of claim 1, comprising plastically deforming the inner layer bysaid radially-inward pressure.
 4. The method of claim 1, where the innerlayer is a thermoplastic, comprising pressing the insert into the innerlayer when the inner layer is at a softening temperature.
 5. The methodof claim 1, comprising maintaining said radially-inward pressure on theinner layer during and after manufacturing the outer layer.
 6. Themethod of claim 5, comprising holding together the parts of the insertunder tension in one or more fastenings that connect those parts.
 7. Themethod of claim 6, comprising press-fitting the parts of the inserttogether to engage the fastenings and to apply radially inward pressureto the inner layer.
 8. The method of claim 1, further comprisingapplying an anti-corrosion layer and an adhesive primer layer onto thepipe lengths before manufacturing the inner layer.
 9. The method ofclaim 1, wherein the inner layer is moulded around the joined pipelengths.
 10. The method of claim 1, wherein the inner layer is wound orwrapped around the joined pipe lengths.
 11. The method of claim 10,wherein the inner layer is extruded in situ.
 12. The method of claim 1,wherein the outer layer is moulded around the insert.
 13. The method ofclaim 1, wherein the outer layer is wrapped around the insert.
 14. Themethod of claim 13, wherein the outer layer is extruded in situ.
 15. Afield joint arrangement for a pipeline, comprising: pipe lengths joinedend-to-end at a joint; thermally-insulating parent coatings on each ofthe pipe lengths, the parent coatings having chamfered edges that arespaced from the joint in mutual opposition about alongitudinally-extending gap; and an annular field joint coating thatcomprises: an hourglass-shaped inner layer that surrounds the pipelengths, extends longitudinally along the gap between the chamferededges and has inclined end parts that at least partially overlie therespective chamfered edges; a thermally-insulating solid insert disposedin the gap, the insert being made up of two or more parts that cooperateto surround the inner layer; and an outer layer surrounding the insertthat forms a watertight barrier and has one or more melted interfaceswith the inner layer.
 16. The arrangement of claim 15, furthercomprising an anti-corrosion layer and an adhesive primer layerinterposed between the pipe lengths and the inner layer.
 17. Thearrangement of claim 15, wherein the parts of the insert are heldtogether by tension to maintain radially-inward pressure on the innerlayer.
 18. The arrangement of claim 17, wherein the parts of the insertare held together by one or more straps under tension.
 19. Thearrangement of claim 17, wherein the parts of the insert are heldtogether by one or more clips under resilient tension.
 20. Thearrangement of claim 17, wherein the parts of the insert are heldtogether by barbs of one of those parts engaged with holes of another ofthose parts, that engagement maintaining resilient deflection of theparts to keep the barbs under tension.
 21. The arrangement of claim 20,wherein the barbs are an interference force-fit within the holes. 22.The arrangement of claim 17, wherein the inner layer is elasticallycompressed by the radially-inward pressure.
 23. The arrangement of claim17, wherein the inner layer has been plastically deformed by virtue ofradially-inward pressure exerted through the insert.
 24. The arrangementof claim 15, wherein the insert is of a softer material than the innerlayer.
 25. The arrangement of claim 15, wherein the inner layer is amoulding formed around the joined pipe lengths.
 26. The arrangement ofclaim 15, wherein the inner layer comprises a tape or side-wrap, woundor wrapped around the joined pipe lengths.
 27. The arrangement of claim15, wherein the outer layer is a moulding formed around the insert. 28.The arrangement of claim 15, wherein the outer layer comprises aside-wrap, wrapped around the insert.
 29. The arrangement of claim 26,wherein the side-wrap is extruded in situ.
 30. The arrangement of claim15, wherein the insert and the inner layer cooperate to fill thelongitudinally-extending gap between the chamfered edges of the parentcoatings.
 31. The arrangement of claim 30, wherein the insert has endfaces inclined to lie closely against the end parts of the inner layer.32. The arrangement of claim 15, having gaps between end faces of theinsert and the end parts of the inner layer, which gaps are filled bymaterial of the outer layer.
 33. The arrangement of claim 15, whereinthe end parts of the inner layer extend longitudinally beyond the outerlayer, which seals against the end parts of the inner layer at meltedinterfaces.
 34. The arrangement of claim 15, wherein the outer layerextends longitudinally beyond the end parts of the inner layer to bondto the parent coatings outboard of melted interfaces with the end partsof the inner layer.
 35. The arrangement of claim 34, wherein the outerlayer bonds to the chamfers of the parent coatings.
 36. The arrangementof claim 15, wherein the end parts of the inner layer extendlongitudinally beyond the chamfered edges to bond to radially outersurfaces of the parent coatings.
 37. The arrangement of claim 15,wherein the end parts of the inner layer terminate on the chamferededges, radially inwardly with respect to radially outer surfaces of theparent coatings.
 38. The arrangement of claim 15, wherein the end partsof the inner layer terminate radially inwardly with respect to aradially outer surface of the insert.
 39. The arrangement of claim 15,wherein the end parts of the inner layer extend radially outwardlybeyond a radially outer surface of the insert.
 40. The arrangement ofclaim 15, wherein a radially outer surface of the insert lies atsubstantially the same radius as radially outer surfaces of the parentcoatings.
 41. The arrangement of claim 15, wherein the end parts of theinner layer terminate radially at substantially the same radius as aradially outer surface of the outer layer.
 42. The arrangement of claim15, wherein the chamfered edges each comprise at least one stepcoinciding with a boundary between layers of the parent coatings. 43.The arrangement of claim 42, wherein an inner layer of each parentcoating protrudes longitudinally into the gap from the chamfered edge.44. The arrangement of claim 42, wherein the end parts of the innerlayer extend into spaces defined by cutting back an outer layer of eachparent coating from the chamfered edge.
 45. The arrangement of claim 15,wherein the insert comprises a longitudinal series of segmentsalternating with relatively flexible links.
 46. The arrangement of claim45, wherein the links are relatively thin parts of the insert leavinggaps between the segments on a radially outer side of the links, whichgaps are filled by material of the outer layer.
 47. The arrangement ofclaim 15, wherein the inner and the outer layers each have a thicknessof between one sixth and one quarter of the aggregate thickness of anassembly comprising the inner layer, the insert and the outer layer. 48.The arrangement of claim 47, wherein the insert has an overall thicknessof between one half and two thirds of the aggregate thickness of saidassembly.
 49. A subsea pipeline comprising at least one field jointarrangement in accordance with claim 15.